A typical magnetic data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks.
The actuator assembly typically includes a coil assembly and a plurality of outwardly extending arms having flexible suspensions with one or more transducers and slider bodies being mounted on the suspensions. The suspensions are interleaved within the stack of rotating disks, typically using an arm assembly (E-block) mounted to the actuator assembly. The coil assembly, typically a voice coil motor (VCM), is also mounted to the actuator assembly diametrically opposite the actuator arms. The coil assembly generally interacts with a permanent magnet structure, and is responsive to a transducer positioning controller.
In a typical digital magnetic data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One of the information fields is typically designated for storing data, while other fields contain track and sector identification and synchronization information, for example. Data is transferred to, and retrieved from, specified track and sector locations by the transducers which follow a given track and may move from track to track, typically under servo control of a position controller.
The head slider body is typically designed as an aerodynamic lifting body that lifts the transducer off the surface of the disk as the rate of spindle motor rotation increases, and causes the transducer to hover above the disk on an airbearing cushion produced by high speed disk rotation. The separation distance between the transducer and the disk, typically 0.1 microns or less, is commonly referred to as head-to-disk spacing or flyheight. As disk storage devices become more sophisticated, flyheights are becoming smaller and smaller.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field emanating from the magnetized locations on the disk.
Conventional data storage systems generally employ a closed-loop servo control system to move the actuator arms to position the read/write transducers to specified storage locations on the data storage disk. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read servo information for the purpose of following a specified track (track following) and seeking specified track and data sector locations on the disk (track seeking).
A servo writing procedure is typically implemented to initially prerecord servo pattern information on the surface of one or more of the data storage disks. A servo writer assembly is typically used by manufacturers of data storage systems to facilitate the transfer of servo pattern data to one or more data storage disks during the manufacturing process.
In one known servo technique, embedded servo pattern information is written to the disk along segments extending in a direction generally outward from the center of the disk. The embedded servo pattern is thus formed between the data storing sectors of each track. It is noted that a servo sector typically contains a pattern of data, often termed a servo burst pattern, used to maintain alignment of the read/write transducers over the centerline of a track when reading and writing data to specified data sectors on the track. The servo information may also include sector and track identification codes which are used to identify the position of the transducer. The embedded servo technique offers significantly higher track densities than dedicated servo, in which servo information is taken from one dedicated disk surface, since the embedded servo information is more closely co-located with the targeted data information.
Pre-embossed rigid thermal (PERT) disk technology uses the thermal response of a magnetoresistive (MR) head induced by servo information on a storage medium in order to position the MR head. As described in U.S. Pat. No. 5,739,972, entitled “Method and Apparatus for Positioning a Magnetoresistive Head Using Thermal Response to Servo Information on the Recording Medium”, issued Apr. 14, 1998 to Gordon J. Smith et al. and assigned to the assignee of the instant application, a PERT disk includes servo information provided to induce a thermal response in the MR head. The servo information is typically provided in the form of pre-embossed surface profile variations on the disk. A controller controls the relative position between the MR head and the embossed disk track using the thermal response induced in the MR head.
Typically in PERT disk technology, a read signal from an MR head is filtered to separate thermal and magnetic components. As disclosed in U.S. Pat. No. 6,088,176, entitled “Method and Apparatus for Separating Magnetic and Thermal Components from an MR Read Signal”, issued Jul. 11, 2000 to Gordon J. Smith et al. and assigned to the assignee of the instant application, the thermal and magnetic components of a MR read signal are separated using a finite impulse response (FIR) filter. The thermal component is the thermal response of the MR head to the surface profile variations on the PERT disk. For the purpose of track following, for example, the surface profile variations may include serrated inner diameter (ID) and outer diameter (OD) track edges, which are radially aligned. For each track, the ID edge serration has a different serration frequency than the OD edge serration. By examining the frequency content of the thermal component of the read signal, the off-track direction and magnitude of the MR head can be determined and an appropriate control signal provided to the actuator to position the MR head over the centerline of a track. This multiple-frequency track serration arrangement provides improved track following without sacrificing data capacity of a disk. Unlike embedded servo techniques, this arrangement does not store servo information in disk space that could otherwise be used for data storage.
Thus, higher areal density can be achieved in varying degrees through the use of technologies such as embedded servo and PERT. However, higher areal density can increase the likelihood of misregistration as each transition is packed closer to adjacent transitions on the surface of the disk. Misregistration occurs when the write head or the read head is not correctly positioned over an intended location on the disk. Misregistration can be caused when the airbearing slider is experiencing unintended motion, for example, due to effects such as vibration, contaminants on the slider's airbearing surface, or contaminants or other topographical defects on the disk's surface. This unintended motion is often referred to as head modulation or airbearing modulation. Airbearing modulation can cause data loss, for example, when a write head overwrites previously written data at an unintended location on the disk. In such a case, both the overwritten data and the newly written data can be lost. In addition, airbearing modulation can cause data errors, for example, when a head reads data at an unintended location on the disk.
U.S. Pat. No. 5,721,457, entitled “Shock Isolation System with Write Inhibit”, issued Feb. 24, 1998 to Sri-Jayantha et al. and assigned to the assignee of the instant application, discloses a computer apparatus that inhibits a write operation based on electrical signals from one or more stress sensors included in shock isolators mounted between an enclosure and a direct access storage device mounted within the enclosure. The stress sensors sense abrupt in-plane motion, e.g., shock or vibration, of the direct access storage device that may cause an airbearing slider within the direct access storage device to experience unintended motion. Unfortunately, the stress senors do not sense other events that can cause unintended airbearing slider motion, such as contaminants on the slider's airbearing surface, or contaminants or other topographical defects on the disk's surface. Moreover, the stress sensors are configured to sense in-plane motion, i.e., motion of the direct access storage device in the plane of the data storage disk, but not normal-to-plane motion, i.e., motion of the direct access storage device along an axis perpendicular to the plane of the data storage disk. Although not sensed by the stress sensors, normal-to-plane motions can also cause unintended motion of the airbearing slider.
In addition, higher areal density can increase the likelihood of data loss if mechanical defects, such as pits, scratches, debris, asperities, lube puddles and dings, are present on the surface of the data storage disk. That is, mechanical defects are more likely to affect transitions written on the surface of the disk as the area of each transition becomes smaller and smaller with increasing areal density. If data is written on a mechanical defect, the read head may not be able to read it successfully. Consequently, it is desirable to detect mechanical defects by performing a glide test of the disk surface during manufacture of the data storage device. Typically, the glide test uses the read head to detect mechanical defects. Disks with mechanical defects are typically replaced. Unfortunately, such glide tests are time consuming (e.g., often exceeding 10 minutes) because the read heads are used to entirely and successively scan the recording surfaces of each disk in the data storage device.
There exists in the data storage system manufacturing industry a need for an enhanced mechanism for addressing the problems associated with airbearing modulation and disk surface mechanical defects. The present invention addresses these and other needs.